The Most Illustrious (Spanish: Ilustrísimo Señor (male) or Ilustrísima Señora (female), literally "Illustrious Sir/Mister") is an honorific prefix that is traditionally applied to certain people in Spain and certain Spanish-speaking countries.
It is a lower version of the prefix The Most Excellent (Excelentísimo/a Señor/a), and was traditionally applied to non-Grandee titled nobles in Spain, but is now used for a series of other offices.
[1][2] The following State and Government officials receive the style "The Most Illustrious": Reference to a Duke in the UK's upper house of Parliament The House of Lords historically employed the prefix "the illustrious Duke"[10] in the late 1800s.
In the 21st century, it has been replaced by the generic prefix "the noble Duke"[11] which is customarily used for all members of the House of Lords, irrespective of their rank.
In other countries, "The Most Illustrious" is rarely used, but rather "Illustrious Highness"